Method of forming electrically insulating chemical conversion coating on aluminum wires

ABSTRACT

1. A METHOD OF FORMING AN ELECTRICALLY INSULATING COATING OF ALUMINA AND CHROMIUM OXIDE ON THE SURFACES OF ALUMINUM OR ALUMINUM ALLOY WIRES WHICH COMPRISES PASSING THE WIRES OF ALUMINUM OR AN ALUMINUM ALLOY THROUGH A CHEMICAL CONVERSION TREATING SOLUTION HELD AT 80 TO 110* C. CONTAINING SODIUM CARBONATE AND SODIUM CHROMATE IN A CONCENTRATION OF 20 TO 200 GRAMS PER LITER OF WATER AND IN WHICH THE WEIGHT RATIO OF SODIUM CARBONATE TO SODIUM CHROMATE RANGES FROM 10:5 TO 10:10 TO FORM AN ELECTRICALLY INSULATING COATING OF ALUMINA AND CHROMIUM OXIDE ON THE WIRES.

United States Patent M US. Cl. 117-218 7 Claims ABSTRACT OF THEDISCLOSURE A method of forming an electrically insulating coatingcomposed of alumina and chromium oxide on the surfaces of aluminum oraluminum alloy wires by passing the wires through a treating solutionheld at 80 to 110 C. of sodium carbonate and sodium chromate in aconcentration of 20 to 200 grams per liter of water and in which theweight ratio of sodium carbonate to sodium chromate ranges from 10:5 to10: 10. The method also comprises applying a layer of a synthetic resinand a layer of paraffin successively onto the chemical conversioncoating to provide insulating aluminum wires having especially goodthermal stability and abrasion resistance and therefore having utilityas magnet wires.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method of forming an electrically insulating chemicalconversion coating on the surfaces of aluminum Wires, and especially toa method of producing aluminum wires having an electrically insulatingchemical conversion coating which are superior in thermal stability andabrasion resistance and have utility as magnet wires.

2. Description of the Prior Art Alumite wires obtained by forming alayer of alumina on the surfaces of aluminum wires, for example, byanodic oxidation have previously found Wide applications in electricmachinery which is desired to be light in weight, such as a liftingmachine, e.g., a lifting magnet. However, the alumina coating of theseWires has high hardness and low bending properties. Therefore, whenAlumite wires of small diameters are bent with a small bending degree,there is the defect that fine cracks are formed in an alumina layer.Furthermore, the cost per unit area incurred in forming an insulatingcoating is high in comparison with the other insulating methods.

It was therefore proposed to form a coating of a mixture of chromiumoxide and alumina on the surfaces of aluminum wires by utilizing such amethod as the MBV method, the Alrok method or the Pylumin method, all ofwhich are known as chemical conversion treatments of aluminum. Accordingto these methods, aluminum wires are immersed in a chemical conversionsolution consisting mainly of a mixture of sodium carbonate and sodiumchromate in a weight ratio of not more than 10:3. By this procedure, acoating can be formed easily, but the thickness of the coating is smalland it has low insulation breakdown voltage. In addition, the adhesionof the coating to the aluminum is poor. AH these defects make thesewires useless as insulating wires.

The present invention provides a remedy to the abovedescribed defects.

An object of this invention is to provide a low-cost 3,846,170 PatentedNov. 5, 1974 SUMMARY OF THE INVENTION After having made various studieson these defects, it was found that since the concentration of sodiumcarbonate is too high in the conventional methods as compared with theconcentration of sodium chromate, a coating formed by sodium chromate isdestroyed by the sodium carbonate, and that because the particlesforming the coating become too fine, the metal surface is prevented fromcontacting the treating solution to inhibit the growth of the coating.Based on these results, it was found that if the concentration of thesodium carbonate is rendered low in comparison with that of sodiumchromate, an insulating coating that can be used for electricallyinsulating wires could be obtained.

The above-described objects are achieved by the method of this inventionfor forming a coating composed of a mixture of alumina and chromiumoxide on the surfaces of aluminum wires which comprises immersing thealuminum wires in a chemical conversion treating solution consistingessentially of a mixture of sodium carbonate (Na CO and sodium chromate(Na CrO in a Weight ratio of from 10:5 to 10:10, preferably 10:6 to10:7. According to this method, the coating formed grows withoutinhibition to form a firmly adherent coating. A suitable concentrationof the mixture of the sodium carbonate and sodium chromate in the aboveratio in the chemical conversion solution is from 20 to 200 g.,preferably 120 to 150 g. per liter of water. If the chemical conversiontreatment is carried out at a temperature of to 110 C., preferably toC., a coating having an insulation breakdown voltage value of at least50 v., which is the minimum value required, can be obtained withinseveral minutes.

Where the coated aluminum wires are to be used as magnet wires whichrequire high degrees of thermal stability and abrasion resistance, asynthetic resin varnish such as an acrylic resin, epoxy resin orsilicone resin varnish is impregnated into the alumina-chromium oxidecoating and dried, and paraffin is coated on the synthetic resincoating.

DETAILED DESCRIPTION OF THE INVENTION The chemical conversion treatingsolution for forming an electrically insulating coating consisting of amixture of alumina and chromium oxide on the surfaces of aluminum wiresshould basically be an aqueous solution containing sodium carbonate andsodium chromate in a weight ratio of from 10:5 to 10:10. Desirably, theconcentration of the mixture in the aqueous solution ranges from 20 to200 g. per liter of water.

If the weight ratio of sodium chromate is less than 5 for each 10 partsof sodium carbonate, the growth of the coating is slow, and in spite ofthe fact that the resulting coating is thinner than expected, it is hardand brittle to an impractical degree. If, on the other hand, the ratioof sodium chromate is above 10 for each 10 parts of sodium carbonate,the rate of growth of the coating is rapid, but the resulting coating isundesirable because of its poor resistance to bending and low value tocorona starting voltage.

When the concentration of the mixture of sodium carbonate and sodiumchromate is less than 20 g./liter, the solution is too dilute, and thereaction is slow. Furthermore, the coating obtained is too hard. If theconcentration exceeds 200 g./liter, the coating obtained is a thicklayer having a high bulk density, but its adhesion to the aluminuinwires is low and this is undesirable for practical purposes.

The chemical conversion treating solution may also contain basicchromium carbonate, basic sodium carbonate, sodium hydroxide or asurface active agent, for example, amphoteric surface active agents suchas an alkyl amino sodium propionate as an amino acid type surface activeagent or an alkyl dimethyl betaine as a betaine type of surface activeagent, in order to stabilize the properties of the solution, or apigment or dye in order to color the coating formed. Especially, theaddition of sodium hydroxide in an amount of from 2 to g., preferably 4to 8 g. per liter of the solution contributes to an increased rate offormation of the coating.

The chemical conversion treatment is preferably carried out at atemperature not higher than the boiling point of the treating solutionbut not lower than 80 C. Treatment at below 80 C. results in slow ratesof reaction and in a coating of the fine particles of alumina andchromium oxide, in which case the resulting coating becomes too high andthe desired thickness of the coating is difficult to obtain. If thetemperature exceeds the boiling point of the solution, pores are formedin the resulting coating due to the bubbles occurring from the surfacesof the wires. Where the chemical conversion treatment is carried out at80 to 110 C., a coating having an insulation breakdown voltage value ofat least 50 v. which is the minimum value required for an electricallyinsulated wire of this kind can be produced within three minutes.

X-ray diffraction analysis of the resulting coating shows that thecoating obtained by this method comprises a mix ture of alumina andchromium oxide in a ratio of about 3:1.

An aluminum plate having a thickness of 1.0 mm. and a width of 30 mm.was treated with the chemical conversion solution in accordance with themethod of this invention, and the properties of the coated plate weremeasured. A coated layer was formed on the same aluminum plate by theconventional chemical conversion treatment and the anodic oxidationmethod, respectively, and the properties of the coated plate weredetermined. The results obtained are shown in Table 1.

Al or an Al alloy. The standard solution components and conditions areas follows:

(1) Solution component G/lit. Potassium dichromate Sodium carbonate (2)Treating condition Same as in a MBV method. 1

It can be seen from the results shown in Table .llthat since the methodof this invention brings about. a higher rate of coat formation wherethe treating timeis fixed, the resulting coating has a large thicknessand a high resistance to voltage per unit thickness, of the coating,which insulation breakdown voltage value is feasible as a coating of aninsulated wire.

The hardness of the coating obtained by the method of this invention issomewhat higher than that obtained with the conventional chemicalconversion methodfWhen the coated aluminum plate was bent, the coatingobtained by the conventional method broke finely and generated finepowders, whereas no change was observed in the coating obtained by themethod of this invention, which demonstrates that the adhesion of thecoating is very superior Since the insulated wires obtained by themethod of this invention contain an insulating coating composed of 'aninorganic material, they have high rigidity and their mechanicalproperties have still something to be desired.

With a view to making up for this insufficiency, a coating of a resinmay be provided on top of the chemical conversion coating obtained bythe method described above. The provision of this resin coating iseffected, for example, by immersing the chemically treated aluminumwires in a solution of polyvinyl alcohol in a concentration of from 50to 100 g./ liter. The degree of saponification and the molecular weightof the polyvinyl alcohol are not limited in particular. A suitablemolecular weight ranges from about 1,000 to 2,500 with a degree ofsaponification of more than 85%.

The desired effects cannot be obtained if the concentration is belowg./liter, and concentrations of above 100 g./liter do not produce theincreased elfects and are TABLE 1 Conventional chemical Method ofconversion method Anodie this inoxidation vention MBV method BV methodmethod Treatment time (min.) 3 5 3 5 3 5 5 Thickness of coating (p) 35 1. 3 2. 0 1. 2 1. 8 8

Insulation breakdown, voltage value (v.)

(superposed) 100 4 1. 0 3 8 200 Pencil hardness (H) 4 5 3 4 3 4 8Resistance to bending (self diameter bending)- Fine powders did Finepowders occurred not occur, and also cracks did not form l Fine powdersoccurred, and also cracks formed.

In the above Table 1, MBV is an abbreviation for modifizierteBauer-Vogel process" and is a surface treating method for imparting ananti-corrision resistance to the surface of Al or an Al alloy. Thestandard treating solution components and treating conditions used inthe MBV method are as follows:

( 1) Solution component 'BV method is an abbreviation for Bauer-Vogelprocess and as the MBV method, is a surface treatment for imparting ananti-corrosion resistance to the surface of uneconomical. By immersingthe aluminum wires in this polyvinyl alcohol solution held at 70 to C.for 1 to 5 minutes, the physical properties of the chemical conversioncoating are remarkably improved, and especially, its insulationbreakdown voltage and hardness are increased markedly. Therefore, thealuminum wires so obtained can:

be used satisfactorily in applications that require a high;

level of these characteristics, such as for magnet wires.

Acrylates and silicone resins can also be used'as resins: in thisinvention. Suitable acrylates which can be employed are those having aviscosity of from 2 to 5 cps. and a-nonvolatile content of from 35 to45% by weight. Suitable silicones are those having a specific gravity offrom 1.0 to 1.1, and a viscosity of from 0.7 to 1.5 cps. Where acrylatesare employed, a suitable treatment temperature is about 80 to C. forabout 1 to 5 minutes and Where silicones are employed, a suitabletreatment temperature from about 120 to C. for about 1 to 5 mitmtesBasically, this invention provides a method of forming a coating ofalumina and chromium oxide on the surfaces of aluminum wires by passingthe wires through a chemical conversion treating solution consistingessentially of a mixture of sodium carbonate and sodium chromate in aweight ratio of from :5 to 10:10 in a concentration of to 200 g./literof water. Where it is desired to impart a higher level of thermalstability and resistance to abrasion, the aluminum wires having thechemical conversion coating are immersed in a synthetic resin varnishsuch as an acrylic resin varnish, and an excess of the varnish issqueezed by felt, followed by drying with hot air to remove the solventin the varnish. Then, the treated aluminum wires are immersed in aliquid obtained by heatmelting solid parafiin, followed by drying. Sucha posttreatment results in a coating having improved mechanical andelectrical properties, especially bending properties and abrasionresistance, which are ascribable to the coated resin, and also, theresin layer adheres firmly to the alumina-chromium oxide coating. Theparaflin layer coated on top of it contributes to an improvement inabrasion resistance, and because of its adherence prevents thedeterioration of the coating caused by the decomposition, evaporation,etc. of the plasticizer or stabilizer contained in the resin varnish.These advantages make it possible to use and store the aluminum wireshaving a chemical conversion coating for long periods of time.

Accordingly, the present invention makes it possible to obtain aluminumwires having a chemical conversion coating which have satisfactoryproperties as magnet wires in regard to electric insulation and thermalstability as well as bending properties and adhesion. In addition, theequipment required to carry out the method of this invention is simplerthan that for the conventional anodic oxidation, and the formation ofthe coating requires little control and is easy and safe. Also, thequantities of electric power and other utilities are small. Therefore,the cost of production of the wires can be made extremely low. Themethod of this invention is especially beneficial for producing aluminumwires having an insulating chemical conversion coating which haverelatively small conductor diameters.

By the term aluminum, as used in the present specification and claims,is meant not only aluminum itself but also an aluminum alloy having amajor proportion of aluminum. Suitable elements which can be present inthe aluminum alloy predominantly of aluminum are silicon, Il'OIl,copper, manganese, magnesium, chromium, nickel, zinc, titanium, etc.,for example aluminum alloys 2017, 2024, 5052, 6061 and 6063 U.S.A.Aluminum Association Code) are suitable. The term aluminum wires ismeant to include not only those having a circular section, but alsothose having a rectangular sectional shape and those in the belt form,which are made of aluminum or an aluminum alloy.

The synthetic resin varnish described above contains synthetic resinhaving a network structure, and is a solution of the resin in a solventtogether with other additives. Examples of the resin are:

Methalack M trademark for an acrylic resin predominantly comprisingmethyl methacrylate, Fujikura Kasei Co., Ltd. Epikote OL trademark foran epoxy resin, having a molecular weight of about 200,000, an intrinsicviscosity in dimethoxy-ethane of 0.39 (di/g.), and a hydroxyl groupcontent of 0.35 (equiv./ 100 g.), Shell Oil Company; and TSR 144trademark for a silicone resin, Color: slightly yellowish andtransparent Specific gravity C.): 1.01:0.02 Viscosity (25 C.) (poise):0.5-1.5 Non-volatile content (percent): 50:2 Film applying thickness(mm): more than 0.03

6 Drying time C.) (hr.): within 1 Volume resistivity (.Qcm.): more than10 Insulating breakdown strength (kv./0.1 mm.): more than 7.0, i TokyoShibaura Electric Co., Ltd.

As the paratlin, solid paratlin -145 specified in JIS (Japan IndustrialStandard) is preferred.

The invention will be described in greater detail by the followingexamples. In the examples to follow, the following testing methods wereused to determine the characteristics of the wires.

(a) Film Thickness: Testing method for enameled copper wire (JIS C 3003)(b) Voltage Resistance: 5

(i) Voltage resistance of two twisted wires (JIS C (ii) Voltageresistance (superposed).

The voltage resistance was measured by superposing two materials, eachof which was electrically connected to a volt meter. A load of 1kg./crn. was placed on the two superposed materials and the voltageresistance was measured.

(c) Pencil hardness (JIS C 3003) (d) Reciprocal abrasion (JIS C 3003)(e) Self diameter winding (JIS C 3003).

EXAMPLE 1 An annealed aluminum conductor (more than 99.7% purity) with adiameter of 0.9 mm. was continuously passed through a chemicalconversion treating bath containing a chemical conversion treatingsolution of the following composition held at 90 C. at a speed such thatit was immersed in the solution for 5 minutes. Then, the aluminumconductor was passed through a wash bath filled with warm water at 80 C.to form an insulating wire having a coating of a mixture of alumina andchromium oxide.

Composition of the Chemical Conversion Treating 1 The test of adhesionusing seli diameter winding was by winding the chemically convertedwire'arouird a'bar'having a diameter equal tothe diameter of the coatedwire and evaluating the surface condition of the chemical conversioncoating.

1 Annealed aluminum wire having a diameter of 0.9 mm.

3 At room temperature.

4At 259 0., 15 nours.

EXAMPLE 2 A semi-hard aluminum (more than 99.7% purity) tape having athickness of 1 mm. and a width of 30 mm. was passed continuously througha chemical conversion treating bath filled with a chemical conversiontreating soluwas only chemical conversion treated. The results are tionof the same composition as used in Example 1, held at shown in Table 4below.

TABLE 4 Insulation break- Reciprocal abrasion down voltage of two Pencilhardness under 200 g. load twisted wires (v.) (H) (times) Room Room Roomtemper- 250 C. x temper- 250 C. x temper- 250 C. x Material ature 15hrs. ature 15 hrs. ature 15 hrs.

0.9 mm. qS, A-Al 1 wire only chemical conversion treated 7O 52 5 5 5 50.9 mm. A-Al wire chemical conversion treated and then post-treated 14075 7 7 10 6 l A-Al-=Annealed aluminum.

100 C. at a speed such that it was immersed therein for While theinvention has been described in detail and 4 minutes, and then passedthrough a wash bath filled with in terms of specific embodimentsthereof, it will be apparwarm water held at 80 C. to produce a tape wirehaving cut to one skilled in the art that various changes and a coatingof a mixture of alumina and chromium oxide. modifications can be madetherein without departing from The characteristics of the tape wire areshown in Table 3. the spirit and scope thereof.

What is claimed is: 1. A method of forming an electrically insulatingcoating of alumina and chromium oxide on the surfaces of aluminum oraluminum alloy wires which comprises passing the wires of aluminum or analuminum alloy through TABLE 3 a chemical conversion treating solutionheld at 80 to 110 mm} C. containing sodium carbonate and sodium chromatein tion a concentration of 20 to 200 grams per liter of water and 5g5}:in which the weight ratio of sodium carbonate to sodium voltage chromateranges from 10:5 to 10: 1-0 to form an electricalliiiiiir 51. 9;). -EggAdhesion (sew 1y insulating coating of alumina and chromium oxide onMaterial ness posed) (H) diameter bending) 1 the WlI'es. I 1/2 55 a 1205 Fine powders did not 2. The method of Claim 1, wherein said methodaddic t lg s o tiogallly compriseis gloating a syriithetic treslnvarnish on sai aumma an 0 romium 0x1 e coa mg. I/IZH Al 5 3. The methodof Claim 2, wherein said method addia..Zlitii";22%???ai'ii iif fiififi ifiii tiJZifJZigifi h ufiili P f a Paraffi Synthetlc the two sides of theUare parallel and separated byadistance equal to the resin varnishcoating. giggeaili gt the bar, the bar also havingadiameter equal to thediameter 4 h method of cl i 1 h i h i h ratio germ-hard aluminum wirehaving a thickness of 1 mm. and a width of sodium carbonate to sodiumchromate in said treating solution is 10:6 to 10:7. ii o i i s fi ii r g40 5. The method of Claim 1, wherein sodium hydroxide is added to saidtreating solution in an amount of 2 to 10 g. per liter of the solution.

6. The method of Claim 2, wherein sodium hydroxide is added to saidtreating solution in an amount of 2 to 10 g. per liter of the solution.EXAMPLE 3 7. The method of Claim 3, wherein sodlum hydroxide is added tosaid treating solution in an amount of 2 to 7 An annealed aluminum (morethan 99.7% purity) con- 10 g, per lit f th l tio ductor having adiameter of 0.9 mm. was passed through R f rences Cit d a chemicalconversion treating bath filled with a chemical UNITED STATES PATENTSconversion treating solution containing g./liter of Na CO 35 g./liter ofNa CrO and 2 g./liter of NaOH 2 .1 4/1973 B9056 et 1 held at C. at aspeed such that it was immersed there- 2,066,180 12/1936 Kmg 148-52 infor 5 minutes to provide an insulated wire having a 2,019,229 10/1935Leah) coating of a mixture of alumina and chromium oxide. 55 2146018972/1949 Meyer The insulated wire was then immersed for 5 seconds in3,119,897 1/1964 P 117-231 X a bath of acrylic resin varnish (MethalackM, trademark, 3,364,030 1/1968 fi Fujikura Kasei Co., Ltd.), andsqueezed with a felt, fol- 3,032,435 5/ 1962 Mlchel 143-627 lowed bypassing for 3 minutes in a drying oven at 80 to 3,059,046 '1'0/1962Westervelt et C. The wire was then passed through a bath of parafiinobtained by heat-melting solid parafiin'l45 speci- 6O RALPH HUSACKPrimary Exammer fied in JIS, squeezed with a felt, and then dried for 1I Us Cl XR minute with air at 30-40 C. The properties of the resultingwire were compared with those of the wire which 117-231; 1486.2, 6.27

1. A METHOD OF FORMING AN ELECTRICALLY INSULATING COATING OF ALUMINA ANDCHROMIUM OXIDE ON THE SURFACES OF ALUMINUM OR ALUMINUM ALLOY WIRES WHICHCOMPRISES PASSING THE WIRES OF ALUMINUM OR AN ALUMINUM ALLOY THROUGH ACHEMICAL CONVERSION TREATING SOLUTION HELD AT 80 TO 110* C. CONTAININGSODIUM CARBONATE AND SODIUM CHROMATE IN A CONCENTRATION OF 20 TO 200GRAMS PER LITER OF WATER AND IN WHICH THE WEIGHT RATIO OF SODIUMCARBONATE TO SODIUM CHROMATE RANGES FROM 10:5 TO 10:10 TO FORM ANELECTRICALLY INSULATING COATING OF ALUMINA AND CHROMIUM OXIDE ON THEWIRES.